The present invention includes methods and devices to compensate for gain and phase imbalance for OFDM and other multi-carrier symbol transmission systems. More particularly, methods and devices for determining compensation parameters are provided. This invention may be applied to a variety of standards utilizing OFDM technology, including IEEE 802.11a, Hiperlan/2 and MMAC.
Orthogonal frequency division multiplexing (OFDM) is a spectrally efficient modulation scheme with application in both wired and wireless communications. It is applied in existing wireless systems including IEEE 802.11a and 802.11g and is proposed for several next-generation wireless systems including IEEE 802.16 and 4th generation cellular. Because OFDM has high spectral efficiency, it is more susceptible to radio impairments. One such impairment, and the topic of this study, is gain and phase imbalance (also known as IQ offset).
Gain and phase imbalance are introduced at both the transmit and receive radios due to the typical variations in mixer components. The D/A and A/D converters may also introduce some gain imbalance; however, this imbalance can be treated as combined with that of the mixer. Gain imbalance occurs whenever the gains in the in-phase and quadrature paths are not identical and phase imbalance occurs whenever the phase separation of the in-phase and quadrature paths is not exactly 90 degrees. Typical gain and phase imbalances of analog mixers are on the order of 1 dB and 5 degrees, respectively. The effect of imbalance is a loss of orthogonality between the in-phase and quadrature paths. This loss of orthogonality can seriously degrade the link performance of an OFDM system, particularly when larger constellations are transmitted.
A major challenge in OFDM transceiver design is ensuring gain and phase balance at the transmitter and the receiver. Standards for OFDM transmission typically lack specific requirements for gain and phase balance, relying instead on performance standards that necessitate good balance. Typically, designers focus on improving the mixer section of the system, to avoid imbalance. This comes at the cost of a relatively expensive mixer section.
Accordingly, an opportunity arises to develop gain and imbalance compensation method and apparatus that improves system performance.